Multiple entries into forest stands are often needed for fire
hazard reduction and ecosystem restoration treatments in the
Inland-Northwest U.S.A. region. However, soil compaction occurring
from mechanized harvesting operations often remains for many
years and may contribute to a decline in long-term site productivity.
A controlled experiment on a silt loam soil was conducted to
determine (a) the effectiveness of logging slash to buffer compaction,
(b) the influence of the number of machine passes, and (c) the
contribution of soil moisture to changes in penetration resistance
during a cut-to-length harvest in northern Idaho. Penetration
resistance was measured at three soil depths (10, 20, and 30
cm) for three different moisture contents (low, medium, and high)
and slash amounts (none, light, and heavy) after each of 12 machine
passes (one pass each with a harvester and an empty forwarder,
and 10 passes with a fully-loaded forwarder). At all three soil
depths the main effect of moisture content and machine passes
on penetration resistance was significant, but slash amounts
alone did not significantly affect penetration resistance. After
12 passes, we measured the greatest penetration resistance in
the medium soil moisture treatment at 5 to15 cm of soil depth.
When evaluated at similar moisture contents after harvesting,
the soil that was driest during machine traffic (low moisture
treatment) had the lowest penetration resistance. Slash was important
for protecting the soil against compaction in the medium and
high soil moisture treatments. Penetration resistance did not
significantly increase after the second pass of a fullyloaded
forwarder (31,752 kg) at any moisture content or slash level.
Managing felling operations to take advantage of dry soil conditions
or using slash when soils are moist may help reduce ruts and
avoid long-term compaction impacts on this soil type.